The present invention relates to a seat belt retractor having a function of winding up a seat belt by means of a motor and to a seat belt device having the same.
FIG. 3 schematically shows a seat belt device installed in an automobile or the like. A seat belt (sometimes called “webbing”) 1 has one end fixed to a side anchor 2 attached to a vehicle floor and the other end wound up by a seat belt retractor 4 through a deflection fitting 3 attached to a vehicle pillar. Though the deflection fitting 3 and the seat belt retractor 4 are generally hidden by the interior panel for the vehicle pillar so that these are normally not seen from the vehicle cabin, these are illustrated as being seen from the vehicle cabin in FIG. 3 for convenience of explanation. The seat belt 1 is provided with tongue 9.
There is another side anchor 5 attached to the vehicle floor to which a strip 6 is attached. Attached to an end of the strip 6 is a buckle 7. After an occupant sits on a seat 8, the tongue 9 is pulled to withdraw the seat belt 1 from the seat belt retractor 4 and to extend the seat belt 1 across the front of the occupant's body and is latched to the buckle 7. As an occupant's hands are then released from the tongue 9 and the buckle 7, the seat belt 1 is wound up by the action of the seat belt retractor 4 such that the seat belt 1 fits the occupant's body.
As mentioned above, the seat belt retractor 4 has a function of taking up a part of the seat belt which is excessively withdrawn when the tongue 9 is latched to the buckle 7 after the occupant sits and withdraws the seat belt 1. To achieve this function, a return spring is used in the conventional seat belt retractor 4. For wearing the seat belt 1, an occupant withdraws the seat belt 1 against the biasing force of the return spring and releases his or her hand from the tongue 9 after latching the tongue 9 to the buckle 7. By the force of the return spring, the part of the seat belt 1 which is excessively withdrawn is thus retracted by the seat belt retractor 4 until the seat belt fits the occupant's body.
As mentioned above, a seat belt retractor is required to satisfy both the following: (1) to securely wind up an excessively withdrawn seat belt until the seat belt fits the occupant's body or, in case that nobody wears the seat belt, to wind up the withdrawn seat belt until the seat belt is securely housed in a housing, and (2) not to impart unnecessary tightness on an occupant's chest normally wearing the seat belt.
In the case of a seat belt retractor utilizing a biasing force of a single return spring, however, when a spring having a weak biasing force is employed as the return spring in order to reduce the pressure to the chest of the occupant during wearing, the biasing force for winding up the belt (for housing the belt) must be poor so that the operationability and the retractability are deteriorated. On the other hand, when a spring having a strong biasing force is employed as the return spring in order to provide sufficient biasing force for winding up the seat belt, there is a problem of increased tightness in chest of an occupant even when normally wearing the seat belt.
In addition, in the conventional seat belt retractor having a built-in single return spring, there is also a problem that the biasing force of the return spring is increased as the withdrawn amount of the seat belt is increased so that increased withdrawing force is required as the withdrawn amount is increased.
As a seat belt retractor capable of solving the aforementioned problems, a seat belt retractor which can wind up a seat belt by a motor or a combination of a motor and a return spring is disclosed, for example, in Japanese Patent Unexamined Publication No. 2001-225720 (incorporated by reference herein in its entirety). Such a seat belt retractor having a mechanism of winding up a seat belt utilizing a motor is also described in Japanese Patent Unexamined Publication No. H11-301407 and Japanese Patent Unexamined Publication No. H11-334533 (both of which are incorporated by reference herein in their entirety).